1. Field of the Invention
This invention relates generally to a magnetic recording disk with pre-patterned surface features of elevated lands and recessed grooves, and more particularly to a method for planarizing the disk surface.
2. Description of the Related Art
Conventional magnetic recording hard disk drives use either horizontal recording wherein the magnetized regions that define the magnetically recorded data bits are oriented in the plane of the recording layer on the hard disks, or perpendicular recording wherein the magnetized regions are oriented perpendicular to the plane of the recording layer. The conventional disk is a “continuous-media” (CM) disk wherein the recording layer is a continuous layer of magnetic material that becomes formed into concentric data tracks containing the magnetically recorded data bits when the write head writes on the magnetic material. The recording layer also includes a pre-recorded pattern of servo sectors that cannot be written over by the write heads and that are used to position the read/write heads to the desired data tracks and maintain the heads on the data tracks during reading and writing. The conventional CM disk has a protective overcoat, typically formed of amorphous carbon, that covers the recording layer and provides a generally smooth planar surface with no surface features. The read/write heads are located on air-bearing sliders that are supported above the smooth disk surface on a thin film of air or “air-bearing” as the disk rotates.
A variation of a CM disk is a “discrete-track media” (DTM) disk, meaning that the concentric data tracks of continuous magnetic material are radially separated from one another by concentric nonmagnetic guard bands. DTM disks are known in the art, as described for example in U.S. Pat. No. 4,912,585. In a DTM disk, the data tracks are typically elevated lands that contain magnetic material and the nonmagnetic guard bands are trenches or grooves that are recessed below the elevated lands. The nonmagnetic guard bands are either formed of nonmagnetic material or contain magnetic material but are recessed far enough below the elevated data tracks to not adversely the readback signals from the data tracks.
In addition to CM disks and DTM disks, magnetic recording disks with “bit-patterned media” (BPM) have been proposed to increase the data density. In BPM disks, the magnetizable material on the disk is patterned into small isolated data islands such that there is a single magnetic domain in each island or “bit”. The single magnetic domains can be a single grain or consist of a few strongly coupled grains that switch magnetic states in concert as a single magnetic volume. This is in contrast to conventional CM disks wherein a single “bit” may have multiple magnetic domains separated by domain walls. To produce the required magnetic isolation of the patterned islands, the magnetic moment of the spaces between the islands must be destroyed or substantially reduced so as to render these spaces essentially nonmagnetic. In one type of BPM disk, the data islands are elevated, spaced-apart pillars that are separated by nonmagnetic trenches or recesses.
CM disks, DTM disks and BPM disks all require servo sectors that are angularly spaced around the disk and extend generally radially across the concentric data tracks. The servo sectors are pre-recorded patterns that cannot be written over by the write heads and that are used to position the read/write heads to the desired data tracks and maintain the heads on the data tracks during reading and writing. In both DTM disks and BPM disks, the servo sectors may be patterns of elevated servo blocks separated by nonmagnetic trenches or recesses. However, CM disks can also be fabricated with servo sectors of elevated servo blocks separated by nonmagnetic trenches or recesses. Thus CM disks, DTM disks and BPM disks may all have pre-patterned surface features of elevated lands and recessed grooves.
There are several methods for fabricating disks with surface features of elevated lands and recessed grooves. In one approach, particularly applicable for BPM disks, the disks are produced by replication from a mold via nanoimprinting. The nanoimprinting process forms not only the isolated data islands in the data tracks, but also the servo blocks in the servo sectors. In nanoimprinting, a mold or template replicates a topographic pattern of surface features onto a polymeric resist coating on the disk substrate. The disk substrate may have a dielectric coating, such as a silicon nitride film. The nanoimprinted resist pattern is then used as a mask for etching the pattern into the silicon nitride film with a fluorine plasma. After etching the silicon nitride, the resist is removed. Magnetic material is then sputter deposited over the lands and grooves. The grooves may be recessed far enough from the read/write heads to not adversely affect reading or writing, or they may be “poisoned” with a dopant material to render them nonmagnetic. Nanoimprinting of BPM disks is described by Bandic et al., “Patterned magnetic media: impact of nanoscale patterning on hard disk drives”, Solid State Technology S7+ Suppl. S, September 2006; and by Terris et al., “TOPICAL REVIEW: Nanofabricated and self-assembled magnetic structures as data storage media”, J. Phys. D: Appl. Phys. 38 (2005) R199-R222.
For disks with pre-patterned surface features of elevated lands and recessed grooves there is need to planarize the surface topography so that the slider is maintained at a relatively constant “fly height” by the air-bearing generated by the rotating disk. U.S. Pat. No. 6,680,079 B2 describes a method of planarizing a disk surface by applying a perfluorinated polyether (PFPE) polymer with a functional acrylate end group, and then curing the polymer to cross-link it and bond it to the protective disk overcoat. However, this method appears to be applicable only to disks with relatively small variations in surface topography. With larger topography variations (in the range of about 10 nm or above), undesirable recession of the polymer in the grooves occurs after curing. Thus, for disks with relatively large variations in surface topography, such as DTM disks and disks with pre-patterned servo blocks, a large variation in surface topography still remains after planarization by this method.
What is needed is a method for planarizing the surface of a magnetic recording disk that has pre-patterned surface features of elevated lands and recessed grooves where there is a relatively large variation in the surface topography.